Could a single 12.5 mg dose of methyltestosterone safely be used as a sexual enhancer for a woman? maybe 2 x wk, 25 mgs total...any possibility of virilizing sides at that dose?


Yep.
The presence of any exogenous male hormone is going to produce some sort of side because the dosing is still going to be significantly more than is naturally produced by the female body.
A single does isn't going to do much of anything. Repeated dosing is cycling, no matter how "small" the dose. The two things I'd probably expect from even a very light cycle - interrupted period (which does NOT mean she can't get pregnant btw) & increased clit / sensitivity (which is probably what you're going after anyway). Most likely some additional acne / oily skin as well. Internally it is also possible to see a bump in blood pressure & cholesterol levels. Addiotionally there might be mood changes as well.
Ain't nuttin for free. Generally I wouldn't recommend cycling strictly for sexual enhancement unless you're working w/ a doc - usually the approach is to go w/ a topical test cream. The thing is that you are choosing to self-medicate w/ male hormones. In the big picture, a small amount is not all that dramatic, however that's a very broad statement and does not account for any specific reaction to whatever compound you're (she's) using. There are always sides when you screw w/ your hormone profile - you can't pick & choose the ones you want or don't want. Its always your own personal experiment and you need to be aware of & responsible for any results AND sides you get from it.
Further you really shouldn't stay on a continued cycle - if its strictly for sexual enhancement, that sort of assumes a maintenance state - and cycling w/ male hormones isn't really a maintenance state, so you'd want to, just like any other cycle, - plan it & stick to it. Come off when you're scheduled to come off. And also be aware of the half-life & detection times to understand how long the effects can be occurring even if you've stopped the cycle or decided you didn't like the sides.